Comparison of a fluorinated aryl thiol self-assembled monolayer with its hydrogenated counterpart on polycrystalline Ag substrates

The effects of perfluorination of aryl thiols on surface coverage, surface electronic properties, and molecular orientation of self-assembled monolayers of thiophenol (TP) and pentafluorothiophenol (F₅TP) on polycrystalline Ag were evaluated using linear sweep voltammetry, ultraviolet photoelectron spectroscopy (UPS), and surface Raman spectroscopy, respectively. Electrochemical reductive desorption by linear sweep voltammetry indicates a surface coverage for the TP monolayer of (5.07 ± 1.29) × 10 ^-10 mol/cm², equating to a molecular area of 32.8 ± 8.3 Ų, and a surface coverage for the F₅TP monolayer of (1.95 ± 0.59) × 10^-10 mol/cm², equating to an area of 85.2 ± 25.8 Å ²/ molecule. TP-modified Ag exhibits a change in work function (δφ) of -0.64 eV relative to bare Ag, whereas F₅TP modified Ag exhibits a δφ of +0.54 eV relative to bare Ag. Quantitative analysis of the UPS and reductive desorption results yields molecular pictures of the proposed interfaces with TP molecules tilted <20° from the surface normal in a herringbone pattern spaced 6.4 Å apart and F₅TP molecules in a more disordered arrangement tilted 67° from the surface normal with an intermolecular distance of 10.4 Å. Qualitative surface Raman spectroscopic analysis of in-plane and out-of-plane modes for these systems confirms that TP molecules are oriented more vertical than F₅TP molecules in these monolayers.